The present invention relates to a disk driving apparatus using a dynamic pressure-type bearing device to be used in an optical disk driving apparatus and a magnetic disk driving apparatus, etc.
Recent optical and magnetic disk driving apparatuses show a tendency towards a compact and light-weight structure with an increased capacity. Spindle motors driving these apparatuses similarly will inevitably be required to be small and thin, and moreover highly accurate, as notebook-size personal computers become wide spread. Also, an improvement in shock resistances is needed. Although ball bearings have been often employed as bearings for spindle motors, small ball bearings adopted to cope with the reduction of an outer diameter of the spindle motor are insufficient to achieve a sufficient rotation accuracy, impeding the fulfillment of the above requirement for a larger capacity and extraordinarily deteriorating shock resistances to generate noises.
Because a large capacity cannot be attained with the rotation accuracy of ball bearings, a dynamic pressure-type fluid bearing spindle motor having a lubricating oil filled therein has started to be put in use, wherein a bearing in a thrust direction is made a pivot bearing.
FIG. 8 shows an example of such a kind of rotary driving apparatus as referred to hereinabove.
A conventional magnetic disk driving apparatus will now be described below with reference to FIG. 8.
FIG. 8 is a sectional view of a conventional magneto-optic disk driving apparatus using a dynamic pressure bearing in a state engaged with a magneto-optic disk.
In FIG. 8, the reference numerals are: 201, a magneto-optic disk; 202, a disk hub; 203, a shaft; 204, a sleeve part; 205, a thrust plate; 206, a chucking magnet; 207, a shaft clamping part; 208, a rotor hub part; 209, a rotor frame; 210, a magnet; 211, a stator core; 212, a coil; 213, a printed circuit board; 214, a housing; 215, a first radial dynamic pressure bearing part; and 216, a second radial dynamic pressure bearing part, respectively.
The rotor hub part 208 loading and positioning the magneto-optic disk 201 is clamped by the clamping part 207 to the shaft 203, which is engaged with the magneto-optic disk 201 while positioning a rotational center of the disk 201 and is rotated at a predetermined revolution number together with the magneto-optic disk 201. The disk hub 202, formed of a soft magnetic material at the central part of the magneto-optic disk 201, is magnetically attracted and secured to the rotor hub part 208 by the chucking magnet 206. The chucking magnet 206 is fixed to the rotor hub part 208. Also, the rotor frame 209, generally in the shape of a cup for forming a magnetic path of the hollow cylindrical field magnet 210 having many magnetized poles, is secured to the rotor hub part 208. The shaft 203 is pressed into the central part of the rotor frame 209, the magnet 210 is bonded at an inner peripheral part of the rotor frame 209, and the rotor hub part 208 supporting the disk 201 and the chucking magnet 206 are caulked at the top ceiling part of the rotor frame, respectively, thereby constituting a rotor part as a whole. The fixing of the shaft 203, magnet 210, and rotor hub part 208, as well as the chucking magnet 106, to the rotor frame 209 may be done In different ways than the above.
Outside an internal cylindrical part of the housing 214 is rigidly set the stator core 211 having the coil 212 wound therearound. The printed circuit board 213 having elements such as ICs, or a printed circuit pattern formed to drive the motor, is fixed to the housing 214. The sleeve part 204 is secured inside the cylindrical part of the housing 214, to which sleeve part 204 the thrust plate 205 is secured.
The above-constructed rotor part is supported by the sleeve part 204 in a radial direction and by the thrust plate 205 in a thrust direction so as to be freely rotatable.
The shaft 203 is rotatably inserted into a hole of the sleeve part 204 which includes the first and second bearing parts 215, 216 with herringbone grooves. The rotor part is fixed at one end of the shaft 203. The other end of the shaft 203 and the thrust plate 205 set at an end of the sleeve part 204 constitute a thrust pivot bearing for supporting the shaft 203 in the thrust direction.
The dynamic pressure-type bearing device constituted as above operates in the following manner.
When the shaft 203 is rotated, a dynamic pressure is generated in the radial direction via an oil owing to the herringbone grooves formed in the bearing parts 215, 216 of the sleeve part 204, thus letting the shaft 203 float and rotate in a non-contact manner. Since the front end of the shaft 203 and the metallic thrust plate 205 constitute the pivot bearing in the thrust direction, the shaft 203 is not floated in the thrust direction, so that the height of a disk surface is not changed between the stationary state and the rotating state.
Although the oil used in the fluid bearing is an insulating oil, the magnetic disk is connected and turned conductive to a chassis of the device because the front end of the shaft 203 and the thrust plate 205 are formed of metal. It can be prevented that the magnetic disc is electrostatically charged during the rotation of the magnetic disk as a result of the friction thereof with the air, and thus a potential difference is generated between the magnetic disk and a magnetic head.
If the sleeve part 204 and the thrust plate 205 are tightly secured by a caulking or the like into a sealed state, the oil is lubricated to the sleeve part 204 secured to the thrust plate 205 thereby to insert the shaft 203 to a set position. However, the thus-sealed state of the thrust part consumes time for the insertion of the shaft 203.
In the prior art arrangement as above, if the shaft 203 is rotary, the dynamic pressure is generated in the radial direction via the oil owing to the action of the herringbone grooves formed in the bearing parts of the sleeve part 204 when the shaft 203 is rotated, floating and rotating the shaft 203 in a non-contact manner. Thus, high reliability is secured. Also in the case where the shaft is of a fixed type, similarly, high reliability is ensured if a dynamic pressure bearing is constituted in the radial direction to thereby rotate the shaft in a non-contact fashion. Since the front end of the shaft 203 and the metallic thrust plate 205 constitute the pivot bearing in the thrust direction, the shaft 203 is not floated in the thrust direction, therefore not changing the height of the disk surface when the rotation of the disk is stopped and when the disk is rotated. However, the sliding motion between the front end of the shaft 203 and the thrust plate 205 brings about abrasion. Specifically, metallic abrasion particles of the thrust plate 205 abraded by the front end of the shaft 203 invade the pivot bearing, accelerating the abrasion. The oil in the dynamic pressure-type bearing device is contaminated, and moreover, the reliability is considerably deteriorated.
If the sleeve part 204 and the thrust plate 205 are securely caulked into a sealed state, the oil is lubricated to the sleeve part 204 fixed to the thrust plate 205 to insert the shaft 203. At this time, the air is sealed inside the sleeve part 204, and an insertion speed for the shaft 203 is consequently related to the amount of the air passing through a gap between the shaft 203 and the sleeve part 204. As such, if the fluid bearing has a narrow gap between them, it inconveniently takes much time to insert the shaft 203 to a set position.